The present invention is related to a method of production of a high strength cold-rolled metal coated steel product.
The present invention is also related to the direct products obtained by the method mentioned here above.
There is a need in the automobile field for cold-rolled hot dip coated steel products having a low yield ratio as well as a tensile strength comprised between 500 MPa and 800 MPa, likewise for steel grades with a high temperature corrosion resistance up to 900xc2x0 C. in combination with good mechanical properties during and after their use at these high temperatures.
Those steels are also commonly called multi-phase steels or preferably dual phase steels.
Document U.S. Pat. No. 4,394,186 is describing dual phase steel sheets having as major constituents a phase being ferrite and at least another phase being either martensite or bainite or retained austenite. These steel sheets have a low yield ratio, of approximately 0.6, and are free from yield point elongation. The production method for obtaining uncoated steel sheets is to heat the steel in a continuous annealing line at a temperature within the intercritical region followed by a quenching in one step (called primary cooling R1) from the annealing temperature to a temperature lower than 200xc2x0 C. with an average cooling rate comprised between 1xc2x0 C. and 30xc2x0 C. per second. The composition of the steel has a carbon content comprised between 0.01 to 0.3% with a manganese content comprised between 0.7 and 1.7%.
The production method for obtaining a hot dip coated steel is to heat the steel in a continuous annealing line at a temperature within the intercritical region followed by a quenching in two steps: in the first quenching step, the strip is quenched (primary cooling R1) down to a temperature between 420xc2x0 C. and 700xc2x0 C. (molten zinc bath temperature) at a cooling rate comprised within the range of 1xc2x0 C./sec less than R1 less than 30xc2x0 C./sec, the second quenching step (secondary cooling R2) consists in a quenching from the molten bath temperature to a temperature lower than 200xc2x0 C. at a cooling rate within the range of 100xc2x0 C./sec less than R2 less than 300xc2x0 C./sec. The first quenching step is to avoid the transformation of austenite to perlite, the second quenching step is performed to obtain the transformation of the austenite into martensite. The described high (between 100xc2x0 C. and 300xc2x0 C. per second) secondary cooling rate (R2) of the steel strip which is still covered with molten metal, is probably feasible at laboratory scale, but in the industrial technology of today this quenching is not feasible. Indeed, after the molten metal coating bath the coated strip (with molten metal at its surface) is cooled in open air (no forced air cooling) during its vertical transfer to the wiping knifes (regulation of the layer thickness) and is then cooled in a vertical cooling device, to ensure the same layer thickness on both sides. A cooling rate, higher than 50xc2x0 C. per second can only be achieved by roll quenching, which is not applicable in said method due to the molten layer, or by water quenching, which is impossible to apply in said method on a molten metal surface and above a molten metal bath. Those two quenching methods are applied on uncoated steel surfaces. So far in the state of the art, no industrial galvanising line has been equipped with such quenching devices used for secondary quenching.
EP-A-0501605 describes a galvanised steel sheet, which has a tensile strength not less than 800 MPa and a yield ratio lower than 0.6. This steel contains carbon, manganese, niobium, titanium and boron and has a dual phase structure. After annealing at a temperature comprised between Ac3xe2x88x9230xc2x0 C. to Ac3+70xc2x0 C. the steel sheet is cooled at a rate higher than 50xc2x0 C. per second down to a temperature comprised between 450xc2x0 C. and 550xc2x0 C. This controlled cooling step should avoid that the perlite transformation occurs. The addition of manganese and chrome as alloying elements as a way of obtaining quenching structures is well known. Those elements have however a very detrimental effect on the adhesion of the coating metal on the steel surface.
JP-A-4350152 describes the manufacture of a galvanised steel sheet having a molybdenum content comprised between 0.005 and 0.5%, a boron-content comprised between 4 and 50 ppm, a silicon-content less than 0.5% and a carbon-content comprised between 0.01 and 0.2% with the presence of some Mn, Al and Ti elements. The annealing temperature at the galvanising line lies higher than Ac3. The cooling is performed at a cooling rate higher than 50xc2x0 C. per second. This method has two main disadvantages: the high annealing temperature of above Ac3 is very expensive and the high cooling rate ( greater than 50xc2x0 C./second) in the secondary cooling, is hardly feasible industrially.
JP-A-56047555 is describing the manufacture of a galvanised steel plate by annealing a cold rolled steel strip through a continuous hot dip galvanising line. The steel composition consists of 0.02-0.07% C, 1.5-2.5% Mn, 0.5-1% Cr, 0.01-0.1% Al, 0.07% or less Si, and the remaining is Fe. The Mn, Cr and C-contents are defined by the following relation:
C+0.06 Mn+0.03 Cr greater than 0.17%. 
The steel strip is soaked between the transformation temperatures Ac1 and Ac3, and soon passed through the hot galvanising bath of the said hot dip galvanising line to obtain the galvanised steel plate having a low yield ratio of approx. 0.7 or less and a tensile strength of approx. 450 MPa or more. The high Mn ( greater than 1.5%) and Cr ( greater than 0.5%) concentrations have such a detrimental effect on the zinc adhesion that it is virtually impossible to obtain a defect free zinc layer for industrial applications. This is due to the heavy manganese and chrome oxides formed at the strip surface before entering the zinc bath.
JP-A-56163219 is describing a cold rolled high-tensile galvanised steel strip whereby a slab of the steel consisting of 0.02-0.15% C, 1.6-3.0% Mn, 0.1-1.0% Cr, less than 0.1% Si, 0.01-0.10% Al and the balance Fe with unavoidable impurities and satisfying the following relation: Mn %+xc2xdCr % higher than or equal to 1.9%, is hot-rolled, pickled and cold-rolled to obtain a cold-rolled steel strip. Then, the slab is heated at an annealing temperature between Ac1 and Ac3 with an in-line annealing type continuous galvanising device and is immediately passed through a galvanising bath, whereby it is plated. The average cooling rates up to the execution of the hot dipping after the in-line annealing are preferably about 2-8xc2x0 C./sec and the average cooling rates down to about 350xc2x0 C. after the plating are preferably about 3-8xc2x0 C./sec. The high Mn ( greater than 1.5%) and Cr ( greater than 0.5%) concentrations have such a detrimental effect on the zinc adhesion that it is virtually impossible to obtain a defect free zinc layer for industrial applications. This is due to the heavy manganese and chrome oxides formed at the strip surface before entering the zinc bath.
Aluminising steel according to the above described process of annealing and cooling in two steps is also a known technique. For high temperature applications, a combination of a good adhesion of the coating, together with a low decrease in strength because of the use at high temperature is necessary. Aluminium coatings on standard commercial sheet steels show a poor temperature corrosion resistance above 650xc2x0 C., because of the formation of brittle Alxe2x80x94Fexe2x80x94Si-compounds.
By adding alloying elements like Ti in the steel, aluminised steel grades have been made commercially available in the past with a high temperature corrosion resistance up to 800xc2x0 C. One commercial steel grade is known to have a good behaviour at 900xc2x0 C. A weakness of those steels is the continuous decrease in strength during the use time, related to the time spent at high temperature. To thwart the decrease in strength in this existing grade considerable amounts of Ti and Nb are added to the steel in order to inhibit the ferrite grain growth. However, by doing this, the decrease in strength is only retarded.
Document JP-A-6057375 is describing an ultrahigh tensile strength steel sheet containing high amounts of Cr ( greater than 0.5 to 1.3%), which is detrimental for obtaining a defect-free metal coating layer.
Document GB-A-1287178 describes a method of manufacturing steel sheet having deep drawability at normal temperatures and heat resistance at high temperatures. High Cr-content equally results in bad quality of metal coating layers, such as obtained by galvanising or aluminising.
Document EP-A-040553 describes a process for producing dual-phase steel, characterised by a low coiling temperature (350xc2x0 C.-580xc2x0 C.) after hot rolling. No specific effort is described to improve the quality of metal coating on the resulting steel sheet.
The present invention aims to produce a cold-rolled hot-dip metal coated multi-phase steel, having a tensile strength of at least 500 MPa, and a yield ratio (Re/Rm) lower than 0.65 in skinned condition and lower than 0.60 in unskinned condition.
The present invention aims to suggest a high strength steel with good formability and good metal coating adhesion behaviour, which are required for instance by the automobile industry for unexposed and exposed parts.
A further aim is to suggest an aluminised steel having a high temperature corrosion resistance up to 900xc2x0 C., good coating adhesion and good strength properties during and after its use at these high temperatures.
The present invention relates to a method and a composition for producing a cold-rolled steel sheet with multi-phase structure and more particularly to a method and a composition for producing a cold-rolled metal coated steel sheet having excellent formability, high strength, low yield ratio and high ductility.
More specific for the aluminised steel the present invention makes it possible to obtain an increase in strength by using it at high temperature, in combination with a good coating adhesion and a low yield ratio. Furthermore, because of the metallurgy of the steel, the mechanical values are reconditioned through its use at high temperature.
The term xe2x80x9cmultiphasexe2x80x9d used here designates that the major constituent phases of the steel are a ferrite phase and a martensite phase. Advantageously in addition of those two phases a low amount of a bainite phase and of a retained austenite phase could be present.
The term xe2x80x9cyield ratioxe2x80x9d designates the ratio: yield strength/tensile strength i.e. Re/Rm.
As a first object, the present invention is more particularly related to a steel composition characterised by:
A C-content, between 0.06 wt % (hereafter denoted as %) and 0.15%,
A Si-content, between 0.1% and 0.4%,
A Mn-content, lower than 1.5%,
A Cr-content, between 0.2% and 0.5%,
A Mo-content, between 0.1% and 0.25%, so that the following condition is met: Cr+2Moxe2x89xa70.7%.
As a second object, the invention also relates to a method of producing a cold-rolled, metal coated, multi-phase steel having the above composition, said method comprising the steps of:
preparing a steel sheet by slab reheating, hot rolling and cold rolling,
soaking said cold-rolled steel sheet at a temperature between Ac1 and Ac3,
performing a primary cooling down to the temperature of the molten metal bath, with a cooling rate, higher than 25xc2x0 C. per second,
performing the hot dip metal coating of said steel sheet,
performing a secondary cooling of said steel sheet to a temperature lower than Ms, with a cooling rate, higher than 4xc2x0 C. per second,
performing a skin-pass reduction between 0% and 0.4%.
As a third object, the invention also relates to the end product, which is a steel product having said composition, which is produced by said method and which is characterised by:
a tensile strength of at least 500 MPa,
a yield ratio lower than 0.65 in skinned condition and lower than 0.6 in unskinned condition.
in the case of the aluminised steel, a temperature corrosion resistance up to a temperature of 900xc2x0 C.